In order to produce toner with appropriate circularity, there has been proposed an apparatus for heat-treating powder particles to spheroidize a shape of toner appropriately. However, in a conventional apparatus for heat treatment, the amount of heat which powder particles receive varies depending upon the position through which the powder particles pass, and hence it is difficult to heat-treat powder particles uniformly.
In order to overcome the above-mentioned problem, an apparatus for heat treatment has been proposed, in which a powder particle supply portion is provided at the center of the apparatus, and a hot air supply portion is provided outside of the powder particle supply portion (see Patent Literatures 1 and 3). Further, in order to heat-treat toner particles uniformly, an apparatus for heat-treatment while rotating an air stream inside the apparatus has also been proposed (see Patent Literature 2).